1. Field of the Invention
The present invention relates to a three-layer antireflective coating composition and process for producing the coating. The invention is excellent in durability, heat-, boiling water-, abrasion-, impact-, and ultraviolet light-resistance, and approaches a high degree of cosmetic uniformity. The antireflective coating has an excellent dyestuff permeability and is easy to clean. The antireflective coating can be applied to objects such as lenses, sheets, and rods and can be applied on a single face or both faces of a lens or a sheet. For each layer of the antireflective coating, the thickness is in accordance with a determined equation involving wavelength in air for oncoming light selected from a visible band.
2. Description of the Prior Art
When an object is viewed through a transparent material such as an optical plastic, if reflected light is intense, a reflected image called a "ghost" or "flare" is produced on the lens. This phenomenon has the result of producing an annoying and unpleasant feeling to the eyes.
A method to reduce reflected light is known. This method involves coating a substrate with a monolayer of film having a lower refractive index than the substrate. It is known that the selection of coating thickness of the material adjacent to the substrate is very important in order to obtain the beneficial reflection-preventive effect. For example, in the case of a monolayer coating film, when the optical thickness of the monolayer is adjusted to 1/4 of the wavelength of the objective ray or an odd number of times thereof, a minimum reflectance and maximum transmittance is obtained. The term "optical thickness" is defined as the product of the refractive index of the coating layer times the thickness of the coating layer.
Formation of multilayer antireflective coatings with proposed selection of thicknesses is also known (see UK Patent 1,417,779, UK Patent 1,406,567, and UK Patent 1,292,717). However, such antireflective coatings are formed by vacuum evaporation deposition. This process poses the following problems in fields of application such as production of antireflective coatings on plastic substrates:
1. A high degree of vacuum results in restriction on the substrate size and type. PA0 2. Manufacturing time is prolonged. PA0 3. Heating to a certain temperature and for a certain length of time may cause distortion of the plastic substrate. PA0 4. Inorganic oxides which are primarily used in layer-forming coating materials, yield reduced adhesion and heat resistance when applied to a plastic substrate. This is due to differences in thermal expansion (the difference in the coefficient of linear expansion between the coating film and the substrate). PA0 5. Dyestuff permeability is completely lost. PA0 6. Productivity is low and the production costs are high.
Other methods for producing antireflective coatings in which use of the vacuum evaporation deposition technique is not used have also been disclosed. These include a method in which a coating containing fine particles is formed (see U.S. Pat. No. 2,536,764) and a method in which an optical element of polymeric material is provided with a microstructured surface (see U.S. Pat. No. 4,114,983 and UK Patent 1,462,618). However, the light transmitted through the transparent material is also scattered, thus transmittance cannot be effectively improved. There is a known method in which a silicon coating is formed on a plastic substrate and then subjected to plasma polymerization to attain an antireflective effect (see U.S. Pat. No. 4,137,365). However, the dyestuff permeability is lost, the productivity is low, and the production cost is high.
An antireflective coating film having a dyestuff permeability, which is formed by treating an organic film containing inorganic fine particles with an activating gas (see U.S. Pat. No. 4,374,158) has been proposed and found to lack heat resistance and water resistance at high temperatures. There is also known an antireflective coating for a solar cell in which the substrate is covered with a liquid two-layer coating. TiO.sub.2 -SiO.sub.2 (titanium dioxide-silicon dioxide)-forming compounds are used for the first layer and SiO.sub.2 (silicon dioxide)-forming compounds are used for the second layer (see Applied Optics, Vol. 18, No. 18, pages 3133-3138). However, this antireflective coating film has no dyestuff permeability and is readily cracked or broken by thermal or mechanical deformation.
Preparation of ultra-thin monomolecular fluorinated siloxanes for antireflective coatings has been described. These films are useful to obtain a surface which is easy to clean, slippery, and protects the inorganic antireflective coating underneath (see JP Patent 61164676 and JP 62148902). However, application of this type of film coating requires an extra step in an already multistep operation. Thus, there is a significant increase in the manufacturing cost.
Moreover, methods to form antireflective films in solution have been described recently (see U.S. Pat. No. 4,590,117, EP Appl. 0119331, Jpn. Kokai Tokkyo Koho JP 59049502, JP 60068319, JP 59049960, and DE 3369568). However, the interface adhesion between the coatings is poor after boiling in water. The weather resistance is also poor and results in a severe loss of abrasion resistance over time. This is especially true for coatings with a high titanium dioxide (TiO.sub.2) content. Fade-meter exposures of 20 hours (see U.S. Pat. No. 4,590,117, page 15, lines 2-5), were considered to be good measures of light resistance, even though the transmittance of the coated sheet did not change. The exposure time was relatively short for a stability test. These problems will be discussed in-depth in the following paragraphs.
Coating instability is manifested in wearing trials, when coatings applied to such substrates as glass or CR-39 lenses, become easily damaged and scratched after heavy exposure to sunlight. (The chemical name for CR-39 is allyl diglycol carbonate, and it is also referred to as optical plastic. Hereinafter, in the specification and in the claims, allyl diglycol carbonate will be designated by the term CR-39 substrate.) In addition, the coatings can wear off from the lens surface due to coating degradation and weakening of the coating surface. This weather sensitivity is due to the highly oxidant character of TiO.sub.2 (in the high refractive index layer(s)) when it interacts with light.
The photooxidation of TiO.sub.2 is well known. The following are examples: a) Photooxidation of binders such as alkyd paints through the formation of peroxy and hydroperoxy radicals (CA79(2):6859q); b) Photooxidation of polyethylene (CA100(24):193058q); c) Photooxidation of hydrocarbons (CA102(18):157786h); d) Photooxidation of waste waters loaded with organic material (CA90(22):174281r); e) Photooxidation of ornithine and putrescine (CA89(3):24751j); f) Photooxidation of water (CA(99(4): 28572f); g) Photooxidation of sulfur dioxide (CA101(16):140933k; h) Photooxidation of cyanide ion (CA(103(10):79294a; i) Photooxidation of ethylene (CA97(13):109378h); j) Photooxidation of alcohols (CA96(9):68081q); k) Photooxidation of olefins (CA95(22):192971y); 1) Photooxidation of commercial polyethylene (CA87(12):85680r); etc.
Another undesirable feature in the above-mentioned patents is the use of an additional hydrolysis step to produce TiO.sub.2 when forming the titanium containing layer. The mentioned patents incorporate an indiscriminate amount of silane monomers in the coating composition. In the present invention, it was determined that a restricted set of chemical compositions is needed to produce coatings that effectively satisfy the requirements mentioned and to produce a maximum transmittance with minimum reflectance. Also, prior patents have claimed two-and three-layer coatings. It was found that two-layer coatings applied to CR-39 lenses have very poor UV resistance, which becomes increasingly worse as the TiO.sub.2 content of the layer adjacent to the substrate increases. We experimented without success with known antioxidants (Irganoxes) and/or UV stabilizers (hindered amines) to try to improve the poor UV resistance.
Additionally, we found that with two-layer coatings in which the first layer (adjacent to the CR-39 substrate) is composed of only TiO.sub.2, the resistance to photooxidation and boiling water was poor. This is due to the fact that a pure TiO.sub.2 layer has very poor adhesion when directly applied to CR-39 substrates, even when these substrates are etched with strong bases. Unlike the present invention, the three-layer coatings claimed in prior patents have a higher refractive index than CR-39 substrate for the bottom layer adjacent to the CR-39 substrate. In attaining this higher refractive index, titanium alkoxides are added to the bottom layer, as well as to the middle layer. Thus, two TiO.sub.2 containing layers are stacked by a final top layer having a lower refractive index than the other two layers. This arrangement has the same poor UV resistance as the two-layer arrangement in which the layer adjacent to the CR-39 substrate contains TiO.sub.2 and is stacked by a top layer having a lower refractive index than the bottom layer.